Jennie L. Borders

Section 14.1 – Properties of Gases Compressibility is a measure of how much the volume of matter decreases under pressure. Gases are compressible because the particles are far apart. Solids and liquids are not compressible.

Factors Affecting Gas Pressure The amount of gas, volume, and temperature are factors that affect gas pressure. Gas pressure is based on the speed and frequency of collisions between the particles and the walls of a container.

Amount of Gas As the amount of gas in a closed container increases, the gas pressure increases because there are more collisions. (directly proportional)

Volume As the volume of a closed container increases, the gas pressure decreases because there is more room for the particles, so they experience less collisions. (indirectly proportional)

Temperature As the temperature of a closed container increases, the gas pressure increases because the particles speed up causing more collisions. (directly proportional)

Section 14.1 Assessment 1. Why is a gas easy to compress? 2. List three factors that can affect gas pressure. 3. Why does a collision with an air bag cause less damage than a collision with a steering wheel? 4. How does a decrease in temperature affect the pressure of a contained gas? 5. If the temperature is constant, what change in volume would cause the pressure of an enclosed gas to be reduced to one quarter of its original value? 6. Assuming the gas in a container remains at a constant temperature, how could you increase the gas pressure in the container a hundredfold?

Section 14.2 – The Gas Laws Poem I wrote to help you remember the gas laws: Boyle is the VIP. Charles likes direct TV. Gay and Lussac will TP the house directly next to me.

Boyle’s Law Boyle’s law states that for a given mass of gas at constant temperature, the volume of the gas varies inversely (indirectly) with pressure. If volume goes down, then pressure goes up. P 1 V 1 = P 2 V 2 P 1 = initial pressure V 1 = initial volume P 2 = final pressure V 2 = final volume Boyle is the VIP.

Sample Problem A balloon contains 30.0L of helium gas at 103 kPa. What is the volume of the helium when the balloon rises to an altitude where the pressure is only 25.0 kPa. 124L

Practice Problems The pressure on 2.50L of N 2 O changes from 105 kPa to 40.5 kPa. If the temperature does not change, what will the new volume be? A gas with a volume of 4.00L at a pressure of 205 kPa is allowed to expand to a volume of 12.0L. What is the pressure in the container if the temperature remains constant? 6.48L 68.3 kPa

Charles’ Law Charles’ law states that the volume of a fixed mass of gas is directly proportional to its Kelvin temperature if the pressure is kept constant. If temperature goes up, then volume goes up. V 1 = V 2 T 1 T 2 V 1 = initial volume T 1 = initial temperature **Remember the change V 2 = final volume temperature to Kelvin T 2 = final temperature K = o C Charles likes direct TV.

Sample Problem A balloon inflated in a room at 24 o C has a volume of 4.00L. The balloon is then heated to a temperature of 58 o C. What is the new volume if the pressure remains constant? 4.46L

Practice Problems If a sample of gas occupies 6.80L at 325 o C, what will its volume be at 25 o C if the pressure does not change? Exactly 5.00L of air at o C is warmed to o C. What is the new volume if the pressure remains constant? 3.39L 8.36L

Gay-Lussac’s Law Gay-Lussac’s law states that the pressure of a gas is directly proportional to the Kelvin temperature if the volume remains constant. If temperature goes up, then pressure goes up. P 1 = P 2 T 1 T 2 P 1 = initial pressure T 1 = initial temperature P 2 = final pressure T 2 = final temperature Gay and Lussac will TP the house directly next to me.

Sample Problem The gas in an aerosol can is at a pressure of 103 kPa at 25 o C. If the can is thrown into a fire, what will the pressure be when the temperature reaches 928 o C? 415 kPa

Practice Problems A sample of nitrogen gas has a pressure of 6.58 kPa at 539K. If the volume does not change, what will the pressure be at 211K? The pressure in a car tire is 198 kPa at 27 o C. After a long drive, the pressure is 225 kPa. What is the temperature of the air in the tire? 2.58 kPa 341 K

Combined Gas Law The combined gas law includes volume, pressure, and temperature. You can use the combined gas law to help you memorize the other gas laws. P 1 V 1 = P 2 V 2 T 1 T 2 P 1 = initial pressureP 2 = final pressure V 1 = initial volumeV 2 = final volume T 1 = initial temperatureT 2 = final temperature

Sample Problem The volume of a gas-filled balloon is 30.0L at 313K and 153 kPa pressure. What would the volume be at standard temperature and pressure (STP)? 39.5L

Practice Problems A gas at 155 kPa and 25 o C has an initial volume of 1.00L. The pressure of the gas increases to 605 kPa as the temperature is raised to 125 o C. What is the new volume in mL? A 5.00L air sample has a pressure of 107 kPa at a temperature of o C. If the temperature is raised to 102 o C and the volume expands to 7.00L, what will the new pressure be in mmHg? 342 mL mmHg

Section 14.2 Assessment 1. How are the pressure and volume of a gas related at constant temperature? 2. If pressure is constant, how does a change in temperature affect the volume of a gas? 3. What is the relationship between the temperature and pressure of a contained gas at constant volume? 4. In what situations is the combined gas law useful? 5. A given mass of air has a volume of 6.00L at 101 kPa. What volume will it occupy at 25.0 kPa if the temperature does not change? 24.2 L

Section 14.3 – Ideal Gas Law The ideal gas law allows us to calculate volume, pressure, temperature, or number of moles of gas. PV = nRT P = pressure (kPa) V = volume (L) n = number of moles (mol) R = ideal gas constant (8.31 L. kPa/mol. K) T = temperature (Kelvin)

Sample Problem A deep underground cavern contains 2.24 x 10 6 L of methane gas (CH 4 ) at a pressure of 1.50 x 10 3 kPa and a temperature of 315K. How many grams of CH 4 does the cavern contain? 2.05 x10 7 g CH 4

Practice Problems When the temperature of a rigid hollow sphere containing 685L of helium gas is held at 621K, the pressure of the gas is 1.89 x 10 3 kPa. How many moles of helium does the sphere contain? A child’s lungs can hold 2200 mL. How many grams of air do her lungs hold at a pressure of atm and a body temperature of 37 o C. (Use 29 g/mol as the molar mass for air.) 251 mol He 2.5g air

Ideal vs. Real Gases An ideal gas follows rules that scientists have created and remains in a gas at any temperature and pressure. (These do not exist) A real gas changes into a liquid or solid at low temperatures and high pressures. Real gases differ most from an ideal gas at low temperatures and high pressures.

Section 14.3 Assessment 1. Under what conditions do real gases deviate most from ideal behavior? 2. What is an ideal gas? 3. Determine the volume occupied by mol of a gas at 15 o C if the pressure is 81.8 kPa. 4. What pressure is exerted by mol of a gas at 25 o C if the gas is in a 0.650L container? 17L 1714 kPa

Section 14.4 – Gases: Mixtures and Movements Dalton’s law of partial pressures states that, at constant volume and temperature, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressure of the component gases. P T = P 1 + P 2 + P 3 … P T = total pressure P 1, P 2, and P 3 = partial pressures of each gas

Sample Problem Air contains oxygen, nitrogen, carbon dioxide, and trace amounts of other gases. What is the partial pressure of oxygen at kPa if the partial pressures of nitrogen, carbon dioxide, and other gases are kPa, kPa, and 0.94 kPa, respectively? kPa

Practice Problems Determine the total pressure of a gas mixture that contains oxygen, nitrogen, and helium. The partial pressures are P O2 = 20.0 kPa, P N2 = 46.7 kPa, and P He = 26.7 kPa. A gas mixture containing oxygen, nitrogen, and carbon dioxide has a total pressure of 32.9 kPa. If P O2 = 6.6 kPa and P N2 = 23.0 kPa, what is P CO2 ? 93.4 kPa 3.3 kPa

Diffusion vs. Effusion Diffusion is the tendency of molecules to move toward areas of lower concentration until concentration is uniform throughout. During effusion, a gas escapes through a tiny hole in its container. Gases of lower molar mass diffuse and effuse faster than gases of higher molar mass.

Graham’s Law Graham’s law of effusion/diffusion states that the rate of effusion of a gas is inversely proportional to the square root of the gas’s molar mass. Rate A = molar mass B Rate B = molar mass A B is always the bigger (more massive) gas. ( ) 1/2

Sample Problem Compare the rates of effusion of helium and nitrogen gas. He effuses/diffuses 2.7 times faster than N 2.

Practice Problems Compare the rates of effusion of sulfur trioxide and bromine gas. Compare the rates of effusion of oxygen gas and carbon dioxide gas. SO 3 effuses/diffuses 1.41 times faster than Br 2. O 2 diffuses/effuses 1.17 times faster than CO 2.

Section 14.4 Assessment 1. In a mixture of gases, how is the total pressure determined? 2. What is the effect of molar mass on rates of diffusion and effusion? 3. What distinguishes effusion from diffusion? How are these processes similar? 4. Explain why the rates of diffusion of nitrogen gas and carbon monoxide are almost identical at the same temperature.

THE END